Aug 9, 2022

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  1. There’s a lot to agree with, but at least as much to puzzle over, in Ian Ashdown’s essay “Circadian Lighting: An Engineer’s Perspective.” Figure 1 is a key visual element of the essay and should be shared far and wide. Ashdown seems surprised that CCT, a metric based primarily on the theoretical physics of blackbody radiators, would be poorly correlated with predicted physiological responses based on the CS model. He then expresses increasing concern about the use of CCT by specifiers to design lighting to address circadian function in people. That’s exactly the point! CCT should never be used for this purpose. Ashdown assumes that a specifier will start and end with CCT as the surrogate metric for circadian effects, and perhaps this is true. Amateur carpenters sometimes use wrenches as hammers or chairs as ladders.

    Ashdown expresses some concerns about the age of the model. Unfortunately, few researchers since 2005 have undertaken direct hypothesis testing to challenge the CS model, despite our plea for such challenges. Perhaps this is because the model is complex. However, Ashdown apparently hasn’t read the papers that have updated and refined the model and its parameters based on new data since 2005. Nonetheless, it is an incomplete model, as all models are. For example, changes in the nocturnal spectral sensitivity of the human circadian system were documented in 2004. Still, most of the hand-wringing we in the IES are so good at when it comes to spectral effects of lighting, is largely unnecessary. Most people need high light levels during the daytime and low levels at night. If white light is used, differences among SPDs are relatively small.

    In the last sentence of Ashdown’s essay he states “Instead, we should simply educate designers on the principles of circadian lighting and trust their judgement.” Without the “instead” this sentence embodies the purpose of the CS Calculator and indeed of the draft UL recommended practice. If anyone is under the illusion that any recommended practice, from UL, IES or anywhere else, can be followed to the letter with a guarantee of successful lighting, please stop. Such documents (hopefully) educate specifiers and arm them with some knowledge to do a better job, but no amount of accreditation from ANSI or anyone else can do more than that.

  2. The essay “Circadian Lighting: A Neuroscientist’s Perspective” by Douglas Steel describes several objections to a draft recommended practice from UL on circadian-effective lighting for day-active people. Many of the objections to the CS model underlying that draft revolve around the perception that the CS model has not been tested rigorously enough. Steel mentions challenges to evolution and climate change and indeed, many of his objections (e.g., “inadequate,” “untested,” etc.) sound similar to objections to those theories. Unfortunately, some of those objections are also at least partly incorrect.

    Steel points out the high precision of an illuminance value of 254 lux and while this certainly is quite a precise number, I can’t find it mentioned anywhere in the UL draft (please correct me if I am mistaken). That the CS model is limited in scope is, I hope, evident by the qualifying statements in the UL draft as well as in the publications describing the model, some of which I have co-written (hence my interest in this essay). All of those publications have undergone peer-review by independent scientists, despite Steel’s contrary claims. In fact we placed these publications in the scientific literature specifically so they could be challenged and tested. They have been challenged, albeit not has much as I would have liked to see, and hopefully will continue to be.

    Steel’s description of the CS model as based on “one or two datasets generated over 15 years ago” is incomplete. In fact, in addition to more recent data, the CS model also uses data from 30 years ago! The status of solid state lighting has nothing to do with the relevance of these physiological data. Steel wonders whether lights having the same CCT could suppress melatonin differently, but shouldn’t – there’s no reason to expect this, as CCT is fundamentally not a physiologically based metric for lighting.

    Steel warns specifiers that using a recommended practice like the UL draft could create “harm” to individuals. Such a statement, if it really needs to be said, needs to be said about every single published application-based recommendation for lighting, including those from IES, CIE or any other body. Does anyone think it is possible to follow any recommended practice legalistically, and thereby to guarantee successful lighting without any negative effects on any individuals? I hope not. That’s not the purpose of such recommendations (even if they are ANSI-accredited!); rather it is to educate specifiers about the inherently imprecise application of light for a multiplicity of benefits. It is fun to study and talk about the specific effects of wavelength, timing, duration and individual differences as they influence specific nonvisual responses to light, and the results of this or that experiment. But in the end, highly precise models that account for specific individual variations according to age, photic history, medical status and other factors are best left to clinicians.

    Decades of field research have illustrated pretty compellingly that high light levels during the day and much lower levels at night are generally beneficial to many people who need to be active during the daytime and asleep at night. Many lighting specifiers and their clients recognize this. Importantly, spectral differences, at least for white light, matter relatively little. Recommended practices for lighting can and should reflect this limited, yet practical knowledge. Steel’s concerns reflect a well-intentioned drive toward scientific completeness. To the extent they are free from errors, I won’t dismiss them as a scientist. But unless the objective is total paralysis, those concerns don’t effectively address the needs of illuminating engineers served by organizations like IES.

  3. With most customers expecting 10 years of carefree service and many industrial applications operating 24/7, do you see the margin for error significant at 80,000 hours?

  4. Back in the ’70’s Dr. Bill Thornton created a recipe of red, green and blue phosphors for fluorescent lamps that resulted in very high visually efficient results in most any daytime or nighttime lighting application. Users claimed they had “more” light, when the light meter stated the opposite. From the chart it shows a large difference in the number of SPDs used, if I have interpreted the chart correctly. Could that skew the results? The chart results also show that the moon and white LEDs provide nearly the same M/P result. Color accuracy (fidelity) under both moon and white LED is less than good as we all have experienced. Don’t we need to combine color accuracy in these attempts to create a more realistic result for visual efficiency, which allows us to work and function is any activity more efficiently? M/P is certainly part of the overall total equation, but is it the only metric to be used?

  5. Naomi, as usual, thank you so much for the description. This helps me out considerably since I have been using the Lucas Tool. I just use my spectrometer and insert the results in the spread sheet and it gives me the M/P ratio. It works and I have never questioned it or dug as deep as you have in the actual measurement.

    I strongly believe this is the future of lighting and specifying based on light and human physiology. CCT just does not do it anymore since the discovery of the ipRGC.

  6. Interesting to see that there is absolutely neither any mention of the Circadian Stimulus / CIa metrics nor any single reference to the huge amount of key contributions by the LRC/Rea/Figueiro front. A debate on CS vs MR would be outside of the scope of this but one of the two major metrics should have been at least mentioned, especially when a scientifically irrelevant Apple and WELL that doesn’t even qualify as a standard are mentioned. I am not affiliated with LRC. My only bias is towards scientific accuracy, scientific relevance and fair citing. Thank you.

  7. This article is a good overview of how lighting will be some part of the Internet of Things.  It does a nice job of pointing out how lighting could either be a driver in this evolving technology ecosystem or may be relegated to a second tier player.  If the lighting industry finds ways to incrementally and quickly add useful features that branch out into the IoT ecosystem then our industry (design & manufacturing) will capture more of the value of this emerging technology.  The lighting industry will definitely need to embrace inter-operable systems in a way that it has not previously done as Dr. Karlicek points out. I would also have pointed out the need to get out of our narrow lighting silo and embrace working with the other system providers on a project.

  8. I really liked how you framed this discussion by “Relating the Gartner Hype Cycle to the Current Situation in the Lighting Industry.” Thank you for your clear and innovative writing on this subject.

  9. Thanks for this detailed look. You did exclude one important dimension, however: photochemistry. My IAQ colleagues point out that these far-UVC lamps cause ozone creation, which is not something one should be breathing. Any device that uses these lamps needs to address this side effect. (Note that this is not a significant by-product of the more common 254-nm germicidal UV lamps.)

  10. Great Details. However you have missed one point – How to take care of Ozone generation. Also The Duration for exposure – how long to stay in the gate for disinfection?

  11. As a practitioner with a great interest in this science, it is great to get a peek under the hood at the thinking behind the many metrics. The challenge, as I see it, is to find the most precise metric that is simple to calculate and utilize when designing a space.

  12. I love Charles’ comment about the challenge: “…simple to calculate and utilize when designing a space”. Having had the opportunity to participate on the UL 24480 Task Force, I’d like to point the reader to the Quick Guide in the document. This is a 2-page, 6-step design guideline “How To” for lighting specifiers and designers. Anyone knowledgeable about lighting will find it easy to follow. For those with less knowledge or time, the other option is: “provide a photopic illuminance of 500 lux in the vertical plane” (mentioned in the article above). Either delivers the much-needed “more light during the. day, and less light at night”, which is where we need to start.

  13. The focus of this article is on “Purpose, Precision, Product.” The author writes, “For the alliterative reason, I am using precision synonymously with accuracy.” Anyone with a scientific background will understand the difference between accuracy and precision. I can express the distance between Los Angeles and New York to within a fraction of an inch – this is precision. To determine this value, I can measure it by applying a straight ruler to a 12-inch globe of the Earth – this is accuracy (or the lack thereof). There are various incarnations of “The Three P’s” in marketing (People, Process, and Product), and motivational speaking (Planning, Practice, and Perseverance), but it is a mockery of the scientific approach to conflate accuracy and precision for the sake of a cutesy title.

    Having been both a professional electrical engineer and an industrial research engineer for forty years, I believe I am qualified in saying that engineers do not “naturally, or through training, gravitate to high precision” (or accuracy). We instead recognize that our profession, regardless of the field, is based on the accumulated knowledge and experience that is embodied in various codes and standards. If anything, we naturally gravitate towards conservative designs that respect public safety.

    But what is “precision” in the context of this article? It appears to be defined as “effort … expended to bridge product to purpose.” I believe that most researchers – certainly myself – would be uncomfortable with the idea of defining research, whether basic or applied, as “product.” A product is generally understood as, “an article or substance that is manufactured or refined for sale.” This implies that there is no scientific or technological uncertainty involved – the antithesis of research.

    Reading further, “precision” is defined as “… the tradeoff between doing the science thoroughly and getting something done.” To be clear, this is in reference to the “purpose” ¬– “to improve building occupants’ quality of life through lighting” and the “product” – “… a guideline for practitioners.”

    If “practitioners” is to be interpreted as professional lighting designers, color me horrified. It is difficult not to see the above “tradeoff” for what it really is: cutting corners in the design process in order to get a commercial product (UL 24489) to market ahead of the competition (i.e., the WELL Building Standard).

    The author continues with, “… it’s time to move past who owns the Product and simply decide what level of Precision is needed to meet our collective Purpose.” The only possible interpretation of this statement is that the Product is UL24880 and the Purpose is a design tool for circadian-based lighting. As I argued in my LD+A Article, both UL24880 and the WELL Building Standard offer unworkable solutions. As for who “owns” the Product, why should any practitioner care?

    I will not debate the various levels of “precision” that may be attained the author’s Circadian Stimulus (CS) metric versus the WELL Building Standard’s Equivalent Melanopic Lux (EML) metric – there is no new information presented in this article, just a rehash of the Circadian Light model that was introduced in 2005.

    The author writes, however, that “… nothing should stand in the way of advancing circadian-effective lighting in buildings.” As the author of “Challenging the Circadian Metric” (LD+A December 2019), I will agree. However, I also stand firm on my argument that both UL 24480 and the WELL Building Standard offer deeply flawed metrics that pay no attention whatsoever to the difference between corneal spectral irradiance and dose (i.e., exposure over time). They completely ignore the occupant’s movement within the space and exposure to daylight over the period of hours.

    Together, vision researchers and building engineers know how to model the corneal spectral dose of an occupant in typical indoor environments. For reasons unknown, however, there has been no interest in developing lighting design software that practitioners can use to model the occupant’s dose over the period of a working day. To base UL 24480 and the WELL Building Standard on spectral power distributions that are empathically not available from luminaire manufacturers and which have horrendous variance from product to product in terms of both Equivalent Melanopic Lux (EML) and Circadian Stimulus (CS) for “high-precision” models, and to further suggest that practitioners implement a “low-precision” model by providing “… a photopic illuminance of 500 lux in the vertical plane, and we’re done” is more than nonsensical – it is an outright insult to professionals who rely on standards organizations to provide useful guidelines and codes based on the best available knowledge and experience.

  14. This may be adding insult to injury, but the Bridgelux whitepaper introduced the metric with, “… presents a new method to make objective comparisons of naturalness between two sources,” followed by an equation for their Average Spectral Difference (ASD) metric. As a colleague noted in a LinkedIn posting some four months ago, this metric is identical in form to the well-known mean absolute percentage error (MAPE) metric, which is a measure of prediction accuracy of a forecasting metric in statistics ( It is one of the most popular measures of forecasting accuracy, and it is recommended in most statistics textbooks.

  15. Thanks, Ian, for this comprehensive look at an important topic. Further to your point about the importance of using daylight when available, readers might be interested in this paper:

    Fahimipour, A. K., Hartmann, E. M., Siemens, A., Kline, J., Levin, D. A., Wilson, H., Betancourt-Román, C. M., Brown, G. Z., Fretz, M., Northcutt, D., Siemens, K. N., Huttenhower, C., Green, J. L., & Wymelenberg, K. V. D. (2018). Daylight exposure modulates bacterial communities associated with household dust. Microbiome, 6(1), 175.